Mystery component solved

["M. E. Foster" <foster_m@swbell.net>]

I guess we can put this one to rest. I think this is item 24 on the wiring
diagram in the Haynes manual, p. 262. To everyone who suggested "starter
relay", I got confused - I thought you were talking about the thing on
p.219 labeled "starter motor relay switch", and to Bob Sykes who provided
the English translation, "lamp test relay", thanks! I now know (I think) what
the most obscure and insignificant part on my car is and what it does.